Cement-based Composite Materials Research Articles

Study on thermal insulation cement and its thermal insulation characteristics for geothermal wells

Reducing the heat loss in wellbore is the key for efficient development of geothermal resource. It is a reliable solution to establish a long-term stable wellbore with good thermal insulation through cementing. In this paper, the cement-based composite thermal insulation material was prepared by using cement as the cementing material, hollow glass beads, foaming agent and stabilizer as main raw materials, and other conventional admixtures. Foams and hollow glass beads can introduce gas with low thermal conductivity into cement, so as to improve the thermal insulation of composite material. Foams are produced by chemical forming process, using foaming agent, which is prepared according electrochemistry and thermodynamics, and the foam stabilizer helps foam distribute in cement slurry stably and uniformly. 10–13% hollow glass beads can significantly reduce the thermal conductivity of hardened cement, without significant adverse effects on the rheology and strength of the material. The thermal conductivity of the composite thermal insulation material can be as low as 0.2998 W·(m·K)−1, which is 62% lower than that of conventional cement, while the compressive strength is 6.10 MPa, meeting the engineering requirement. A thermal-conductivity prediction method is proposed correspondingly based on Maxwell model, and the prediction error of the newly established model is within 2%. This research can provide technical support for efficient development of geothermal resources.

Development of strain hardening cement based composite for concrete retrofitting purposes

The ever-increasing population in earthquake-prone areas necessitates the modification of both the current buildings and those that have already been damaged. Installing earthquake-resistant parts is required for this. In order to fulfill the continually expanding need for retrofitting materials that are both efficient and economical, civil engineers are under great pressure to develop novel new materials. A recent development in the construction sector is cement composites that harden under stress. Small particles and fibers are both used to make these composites. The chemical may enter the creep of the damaged structure without causing any vibrations. In order to restore the structure's original virgin strength, the mechanical and durability properties of the cement-based composite with strain hardening have yielded positive results. These qualities contributed to the success. The SHCC has also been used to connect deck slabs since the addition of fibers boosts the material's potential for abrasion resistance. These characteristics are provided through the incorporation of fibres. The combination of glass-fiber-reinforced polystyrene, (Ground Granulated Blast furnace Slag) and polyvinyl alcohol (PVA) fibers impacts the characteristics of a strain-hardening cement-based composite material has been investigated via experimentation, study, and analysis. A Ground Granulated Blast furnace Slag (GGBS) composition that varies from 10 to 30 percent has taken the place of the prior level of fly ash. PVA fibers with content levels of 2.0 and 3.0 percent, each measuring 12 mm in length, were employed in the experiment. These tests examined the compressive strength, tensile strength, and durability of a wide range of materials. The test findings reveal that although tensile strength and durability characteristics of the strain-hardened cement-based composite rose with a higher percentage of Ground Granulated Blast Furnace Slag (GBBS) content, compressive strength did not change significantly. It has been shown that better outcomes are obtained when PVA fiber content is greater. This is valid for both the mechanical strength and durability characteristics that different combinations have.

Evaluation of degradation and mechanical parameters and sustainability indicators of zeolite concretes

The continued need of reducing greenhouse gas emissions is the main motivator for the research related to the design of new cement-based composite materials. A sustainability indicator used in this study is a suitable tool to compare a group of concrete mixes based on their mechanical and degradation properties, together with information on the production and environmental value of each raw material. The study shows the results of the analysis of two groups of concretes with the replacement of the aggregate by zeolite. Experimental data were taken from a published study and numerical evaluations were made. The two groups differ in the value of the water-cement ratio, 0.44 and 0.40 respectively. The other additional cementitious materials (SCMs) used were volcanic pumice, ground granulated blast furnace slag grades 100 and 120, silica fume and metakaolin. The results of six and sixteen concrete mixes incorporating reference concrete made of Portland cement are compared. Compressive strength and diffusion coefficient results were obtained at 28, 56, and 91 days after concreting. The values of the sustainability indicator are determined in these times and ultimately allow the sorting of all the analyzed mixes. The results show that the use of zeolite alone in concrete is not always advantageous but rather depends on the specific w/c. Among many other interesting results, one of the most important is the sustainability index value (1.99) for concrete combining 15% of zeolite and 35% ground granulated blast furnace slag of grade 120, so almost twice as good as the reference concrete. Among concretes with only zeolite as SCM, 35% of the replacement is the most suitable choice. Furthermore, the combination with other SCMs can produce a significant positive effect in terms of sustainability.

Bending Analysis of Multiferroic Semiconductor Composite Beam towards Smart Cement-Based Materials

A beam-like structure of antisymmetric laminated multiferroic piezoelectric semiconductor (LMPS), which consists of two piezomagnetic (PM) and two piezoelectric semiconductor (PS) layers is proposed. The structure could be in pure flexure deformation under an applied magnetic field. Through this deformation mode and the induced polarization field through the magneto-electro-semiconductive (MES) coupling mechanism, the semiconducting properties of PS layers can be manipulated by the applied magnetic field. In order to better understand and quantitatively describe this deformation mode, the one-dimensional governing equations for the LMPS beam are developed based on the three-dimensional theory. The analytical solutions are then presented for the LMPS cantilever beam with open-circuit conditions. The multi-field coupling responses of the LMPS cantilever beam under the longitudinal magnetic field are investigated. Numerical results show that the amplitude of each physical quantity is proportional to the applied magnetic field, and the thickness ratio of the PS phase plays a significant role in the MES coupling behaviors of the LMPS beam. The proposed structure can be integrated into cement structures but also fabricated cement-based multiferroic PS composite materials and structures. It provides an important material and structure basis for developing structural health monitoring systems in the fields of civil and transportation infrastructures.

Permeability coefficient of pervious cement mortar measured by the constant head and falling head methods

The modified previous concrete and cement mortar, known as previous cement mortar (PCM), is designed with a specific composition to create pores. The coarse aggregate is removed to form smaller pore sizes. PCM acts as a water filter, needing higher permeability than cement mortar but less than previous concrete. Its pores drain water while trapping impurities. This study compares the effects of sand-to-cement ratio (S/C), specimen thickness, and age on permeability and porosity. It also contrasts PCM's permeability coefficient determined by constant head and falling head methods. Numerous studies compare permeability coefficients in previous concrete using these methods, but not for finer aggregate cement-based composite materials like previous cement mortar. PCM uses fine aggregate (0.6-0.85 mm) at 3, 5, and 10 cm thickness with S/C ratios of 4 and 5. Findings show that S/C 5 specimens have significantly higher porosity than S/C 4. The S/C ratios notably impact permeability; the higher ratio means the higher permeability. Permeability coefficients for S/C 4 ranging from 0.006-0.075 cm/s, while S/C 5 ranging from 0.010 to 0.147 cm/s. The relationship between the permeability coefficient between the constant head and falling head methods at the age of 90 days specimen are Kc = 1.0516 Kf (S/C 4.2) and Kc = 0.9325 Kf (S/C 5.2). According to these findings, finer aggregates result in a significantly smaller permeability, to the extent that the constant head method is more reliable compared to the falling head method.

Effects of Strain Rate and Fiber Content on the Dynamic Mechanical Properties of Sisal Fiber Cement-Based Composites

In this paper, a split Hopkinson pressure bar (SHPB) was used to investigate the dynamic impact mechanical behavior of sisal fiber-reinforced cement-based composites (SFRCCs), and the microscopic damage evolution of the composites was analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS). The results show that the addition of sisal fibers improves the impact resistance of cement-based composite materials. Compared with ordinary cement-based composites (OCCs), the SFRCCs demonstrate higher post-peak strength, ductility, and energy absorption capacity with higher fiber content. Moreover, the SFRCCs are strain rate sensitive materials, and their peak stress, ultimate strain, and energy integrals all increase with increasing strain rate. From the perspective of fracture failure characteristics, the failure of OCCs is dominated by the brittle failure of crystal cleavage. In contrast, the failure mode of the SFRCCs changes to microscale matrix cracks, multi-scale pull-out interface debonding of fibers (fine filaments and bundles), and mechanical interlock. This research provides an experimental basis for the engineering application of high-performance and green cement-based composites.

ბეტონების კუმშვა-გაჭიმვის ცოცვადობის ბირთვის უნივერსალური მოდელი

The study of the rheological properties of cement-based composite materials under different conditions of loading and impact is necessary for the development of a complete theory of structural calculation and corresponding improved calculation methods. The article examines the dependence of the rheological characteristics of concrete on the moisture content and age of the materials. Short-term and long-term (creep) tensile tests of samples of traditional (concrete, fine-grained concrete) and new (steel-fiber concrete, basalt-fiber concrete) composites have been carried out. Specimens were saved and tested in a “standard” environment – air relative humidity  100% , temperature 0 T C   20 1 . 0 t  3,7,14, 28,60 and 180-day specimens’ creep were tested. 0 t  28 -day specimen were received standardly. Creep experiments duration 0 ( ) t t  was 180 days. Research data defined creep nucleus of various moisture content and age composites. It was found that the dependence of the creep cores on the age of the materials is well reflected by the qualitative function. An image of the function is developed and the magnitude of quality indicators for composites is determined. An image showing the dependence of the core age of the compressive-tensile creep of composites was created. As a result of the conducted research, a new type of universal model of compressive-tensile creep cores of cementbased composites was created, which takes into account the age and moisture content of the materials.

Use of Vine Shoot Waste for Manufacturing Innovative Reinforced Cement Composites

Due to the current concern with the environmental impacts produced by the construction industry, many studies have been conducted to capitalize on the advantages of waste to develop sustainable materials. The study reports an innovative, non-conventional cement-based composite material containing 10 wt.% vine shoot waste, representing a step forward toward the use of this lignocellulosic waste. The investigations were carried out using scanning electron microscopy with energy dispersive X-ray analysis, Fourier-transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction and solid-state 27Al and 29Si nuclear magnetic resonance spectroscopy. The addition of vine shoot waste to the cement paste increases the amount of hydration products (calcium silicate hydrate (CSH), calcium hydroxide (CH), calcium carbonate (CaCO3) and ettringite), leading to a more compact and dense structure. The structural characterization techniques also confirmed the formation of a higher amount of hydration products in the case of vine shoot waste added to the cement paste.

Uniaxial tensile constitutive relations of basalt textile-reinforced concrete with different grid sizes

Textile-reinforced concrete (TRC) is a widely used cement-based composite material reinforced with textile that can be utilized effectively in the construction of thin and lightweight structures. However, its mechanical properties are greatly affected by changes in grid size. In this research, four groups of 48 specimens with different grid sizes are subjected to uniaxial tensile tests on basalt TRC (BTRC) sheets to study the effect of grid size on the tensile performance of BTRC and analyze the failure modes of BTRC sheets at macro- and meso-scales. The Aveston–Cooper–Kelly (ACK) model and finite element simulation are combined to verify the applicability of the ACK model to the stress–strain constitutive relationship equation in the pre-fracture stage of the BTRC sheet underuniaxial tensile test. With the triple linear model, the bond–slip constitutive equation of the whole process of the uniaxial tensile test on the BTRC sheets is deduced. Results show that the BTRC sheets exhibit obvious strain-hardening characteristics under tensile load. The failure mode of the BTRC sheets is typical debonding failure, and the smaller the grid size is, the more microcracks are developed by a single crack. The basalt textile cannot significantly improve the cracking strength of the concrete matrix, but it can effectively increase the ultimate tensile strength of the component. A small grid size equates to good tensile strength and bonding behavior and therefore endows the material enhanced ductility. The bond–slip constitutive equation for the whole process of uniaxial stretching of BTRC sheets is established, and the ACK and triple linear models show good applicability to the solution of the constitutive relationship of the BTRC sheets’ uniaxial tension.

Development of reduced thermal conductivity ductile cement-based composite material by using silica aerogel and silane

Multi-function cement-based materials will show stronger competitive than traditional single-function building materials. Aerogels have attracted widespread attention in cement-based insulation materials because of their extremely low thermal conductivities. However, neither the compatibility between aerogels and matrix matrices nor the strength degradation of cement-based materials with aerogels has been effectively resolved. Therefore, this work aimed to improve aerogels' application prospects in cement-based materials and realize the functional diversity of cement-based composite materials. In this study, high-ductility cement-based composite materials served as the matrix while using aerogel as a lightweight aggregate to replace an equal portion of sand from the original cement. A reduced thermal conductivity ductile cement-based composite (RTCDCC) material was prepared by directly mixing silane into the cement-based material. The effects of the aerogel and silane on the fluidity, strength development, bending performance, thermal conductivity, and density of the slurry under the combined action of the aerogel and silane were systematically studied. In addition, the specimens were tested by nitrogen adsorption, X-ray diffraction, and scanning electron microscopy to investigate the pore characteristics, chemical compositions, and microstructures of the hydration products. The results demonstrated that direct mixing of silane into cement-based materials could improve the degree of bonding between the aerogels and cement-based materials, reducing the adverse effects of mixing these materials and optimizing the pore structure. In addition, when the aerogel content reached 20% with a silane content of 0.8%, the specimen exhibited favorable mechanical properties and ductility with a significantly decreased thermal conductivity.

A large-sized thermoelectric module composed of cement-based composite blocks for pavement energy harvesting and surface temperature reducing

Roads carry a lot of green and renewable energy. Thermoelectric cement as a green material can convert thermal energy to electrical power by using the temperature difference between the road surface and the underground. Cement-based materials can meet the needs for large-scale application in pavements because of their wide source, low cost, and good mechanical properties. Due to the preparation of large-size thermoelectric cement materials, the thermoelectric performance will be reduced, but the existing thermoelectric cement materials with excellent thermoelectric performance are small in size and they cannot be used for large-area applications. It is still a problem that thermoelectric properties of large-size thermoelectric cement-based composites prepared by the traditional pouring method are prone to dry shrinkage crack. Herein, to solve the problem of poor thermoelectric properties of large-sized thermoelectric cement materials. A thermoelectric cement block prepared by dry pressing is reported to be assembled into a large thermoelectric module of cement-based composite more suitable for pavement energy harvesting. The thermoelectric cement block is 50mm × 50mm × 20 mm, and the size of assembled into a large thermoelectric module is 300mm × 300mm × 35 mm. The addition of expanded graphite and metal oxides raised the average conductivity of the cement blocks to 0.63 S/cm and the average Seebeck coefficient to 20.79 μV/K, respectively. Tests have found that using a series connection of thermoelectric conversion modules can yield higher energy. Compared to other pavement thermoelectric generator (TEG) energy harvesting systems, the thermoelectric modules are less expensive and allow for large areas to be laid on the pavement. The module assembled using thermoelectric cement-based composite materials can generate 0.5 kW h of energy for 24 h on a road with a length of 1 km and a width of 10 m. Meanwhile, the thermoelectric module of cement-based composite has the ability to reduce the surface temperature and mitigate the urban heat island effect. The test results showed that it can reduce the temperature by 1°C–3°C. The results of this study provide a reference for future large scale practical applications of thermoelectric cement-based composites.

The Use of Electrochemical Impedance Spectroscopy as a Tool for the In-Situ Monitoring and Characterization of Carbon Nanotube Aqueous Dispersions.

So far, there is no validated technology for characterizing the dispersion and morphology state of carbon nanotubes (CNTs) aqueous dispersions during sonication. Taking advantage of the conductive nature of CNTs, the main hypothesis of the current study is that Electrochemical Impedance Spectroscopy (EIS) is an appropriate technique for the in-situ monitoring and qualification of the dispersion state of CNTs in aqueous media. To confirm our hypothesis, we monitored the Impedance |Z| during the sonication process as a function of type CNTs/admixtures used for the preparation of the aqueous solutions and of crucial process parameters, such as the applied sonication power and duration (i.e., sonication energy). For dispersions above the percolation threshold, a drop of |Z| by approximately seven orders of magnitude was observed, followed by a linear reduction. The dramatic change in |Z| is regarded as an indication of the formation of a conductive path or destruction of an existing one during sonication and can be used to characterize the dispersion and morphology state of CNTs. The results of the EIS provide, straightforwardly and reliably, the required information to create an optimum dispersion protocol for conductive CNT suspensions. The produced dispersions are part of research focusing on the manufacturing of cement-based composite materials with advanced thermoelectric functionalities for energy harvesting. Such dispersions are not only limited to energy harvesting applications but also to applications where functionalities are introduced through the use of conductive-based suspensions.

Effect of Fiber Type and Volume Fraction on Fiber Reinforced Concrete and Engineered Cementitious Composite Mechanical Properties

Engineered cementitious composites (ECC) are an ultra-ductile cement-based composite material reinforced with short randomly distributed fibers. It differs from fiber reinforced concrete (FRC) in that it has a distinct ductile behavior. The study aims to assign mechanical properties, such as tensile, flexural, and compressive strength using locally available fiber rather than polyvinyl alcohol (PVA) fiber, which is not widely available in many countries, to ECC. PVA fiber is also very expensive. Instead of PVA, lightweight fibers, such as polypropylene, polyolefin, and glass fiber, as well as heavyweight fibers, such as steel fiber, were used. To assess the mechanical properties, the influences of curing, fiber volume fraction (2%, 4%, and 6%), fiber type, and fiber hybridization were adjusted in this study. The formation of multiple cracks along the specimen is the governing factor in ECC formation. The test results show that increasing the fiber volume fraction improves flexural and tensile strength. Water curing increased compressive, tensile, and flexural strength. Lightweight fiber hybridization has no effect on compressive strength, whereas heavyweight fiber hybridization improves compressive strength. For tensile and flexural strength, hybridization was associated with an improvement in all mechanical properties. The hybridization of lightweight fiber achieved ECC behavior at a lower volume fraction than the use of a single fiber volume. Relationships between tensile strength and flexural strength depending on the compressive strength of ECC were driven by demonstrating high performance.

Flexural Properties of Textile-Reinforced Concrete with Nonorthogonal Grids

Textile-reinforced concrete (TRC) is a cement-based composite material that uses textile as a reinforcement material. The weft-direction fiber bundles in the traditional orthogonally arranged warp-weft textile hardly bear force, and its bonding strength with the weft fiber bundle is not ideal. Under the action of force, a small included angle between the stressed fiber bundle and the stressed direction can effectively increase the anchoring effect of their fibers in the matrix, resulting in higher bonding and reinforcement efficiency. To improve the utilization rate of fibers and the bonding strength between the textile and the concrete matrix, an arrangement along the diagonal of the grids was proposed in this paper. The flexural properties of basalt TRC plates with orthogonal grids (OG-BTRC) and plates with nonorthogonal grids (NOG-BTRC) with different grid angles and grid sizes with different laying methods, namely, a side layout (SL) and diagonal layout (DL), were studied through four-point bending tests. A comparative analysis was carried out with an ABAQUS simulation and the test results. The results showed that with a decrease in the grid angle, the BTRC specimens gradually showed a failure mode of multiple cracks, and most of the cracks appeared in the pure bending area; as the grid angle decreased, the BTRC specimens exhibited excellent flexural bearing capacity, good ductility, and high toughness. The total number of cracks on the specimen increased when it failed, while the spacing of the cracks decreased, and the fracture morphology appeared as fine and uniform features. The toughness of the specimen with a small grid angle using the DL laying method was greater than that using the SL laying method. The software simulation value matched the test data well, which proved that the test result was reliable.

Optimization Effect of Metakaolin on Macro- and Micro-Mechanical Properties of Composite Cementitious Materials under Different Curing Conditions

To provide the theoretical basis for the engineering application of metakaolin as aluminum-rich pozzolanic ash materials, the promoting effect of metakaolin on the macro- and micro-mechanical properties of composite cement-based composite materials was explored under high-temperature steam and standard curing conditions. Analysis techniques, which involved thermogravimetric and nanoindentation coupled with scanning electron microscopy-energy dispersive X-ray spectroscopy, were used. To analyze the experimental data, the ACI empirical formula and the copula function were used. The correlation among the hydration degree of the MK-cement cementation system, the Al/Si of the C-(A)-S-H phase, and the nanomechanical properties of the C-(A)-S-H gel phase was investigated. According to prior research, the macroscopic mechanical properties and the substitution rate of the MK-cement composite cementitious system can be improved under high-temperature curing conditions in a short period, in which the optimum substitution rate of metakaolin is 20%. The ACI empirical formula was used to demonstrate the functional relationship between the metakaolin replacement rate, curing time, and compressive strength. The MK-cement cementation system can eliminate the defect phase, reduce the CH phase content, and then increase the C-(A)-S-H gel phase content and bulk density. The micro-mechanical properties of the C-(A)-S-H gel phase rises due to its phase content and Al/Si ratio. Furthermore, the copula function verifies the dependence of the nanomechanical properties of C-(A)-S-H gel and Al/Si.

Investigation of two different water-dispersed graphene on the performance of graphene/cement paste: Surfactant and superplasticizer effect

• The compressive strength of graphene-based cement paste with 0.1% SP was increased by 82.05%. • Surfactants did not show better performance in the hydration process due to their hydrophilicity. • The optimized concentration of graphite and dispersing agents is determined. • The graphene produced by the superplasticizer has the lowest level of defects. • The microstructure results are in accordance with compressive resistance results. The excellent performance of graphene-based materials to use in concrete has recently attracted the attention of civil engineering researchers. However, the application of graphene faces obstacles due to the high cost of graphene and agglomeration phenomena in cementitious composites. As a result, research into the production of water-dispersed graphene, a combination of graphite and a dispersing agent, is a particularly cost-effective alternative. In this regard, it is essential to select a dispersing agent capable of effectively separating graphene layers while also improving mechanical properties and impermeability. In this study, the influence of two different dispersive agents, surfactant (S) and superplasticizer (SP), on mechanical properties, permeability, and microstructure of cement paste was investigated. SEM, TEM, and Raman spectroscopic analyses demonstrate that multilayer graphene sheets made using superplasticizer have higher quality and fewer defects than surfactant-based sheets. After 3 days of aging, the compressive strength of graphene-containing cement paste with 0.1 % superplasticizer increased by 82.05 % compared to the control paste. On the other hand, a decrease in sorptivity by 54 % compared to the control paste is observed by graphene-containing cement paste with 0.56 g/l surfactant. The microstructure investigation revealed that graphene accelerates the CH and C-S-H productions due to the nucleation effect in the composite cement matrix; however, the effect of SP is more significant than S in cementitious composites. This research suggests that water-dispersed graphene generated by 0.1 % SP and 0.56 S g/l has the potential to be used as an innovative admixture in cement-based composite materials.

Photocatalytic Cementitious Material for Eco-Efficient Construction—A Systematic Literature Review

Photoinduced processes governed by light activated TiO2 have been studied in many ways. One of the most active areas is the development of TiO2 photocatalysis technologies on their application for reducing environmental impacts. The immobilization of TiO2 on solid support, such as cementitious materials, greatly enhances its use in practical applications. In this review, a wide range of applications for achieving eco-efficient building using cementitious composite materials containing TiO2 photocatalyst was presented. The basic mechanism of photocatalysis, such as electron excitation, charge transfer process, reactive oxygen species (ROS) generation, and its role to oxidize the pollutant and microorganisms were extensively discussed. Unlike self-cleaning and air purification systems, the study on the antibacterial function of a cement-based surface containing TiO2 is very limited. In photocatalytic cementitious materials, the key element affecting the photocatalytic performance is the accessible active surface area. However, microstructure of cementitious materials changes with age due to hydration and surface carbonation. Hence, surface area reduction and mass transfer limitation become the main drawbacks of incorporating TiO2 in cementitious materials. This review, therefore, provides the state of the art in photocatalytic cement-based composite materials and identifies the areas in which future improvement is needed.

Experimental Study on the Shear Behavior of Reinforced Highly Ductile Fiber-Reinforced Concrete Beams with Stirrups

The aim of this study is to improve the shear behavior of reinforced concrete (RC) beams with stirrups by using highly ductile fiber-reinforced concrete (HDC), which is a fiber-reinforced cement-based composite material with tensile-strain-hardening properties. Twelve reinforced HDC (RHDC) beams and three RC beams with stirrups were tested under a concentrated load. The experimental parameters involved the shear span to effective depth ratio, stirrup ratio, and longitudinal reinforcement ratio. The results revealed that the mode of failure of RHDC beams, which exhibited better ductility than RC beams, included diagonal compression, shear compression, diagonal tension, and flexural shear failure. RHDC beams exhibited stable multiple crack propagation behavior and satisfactory integrity, thus showing that HDC effectively restricted the development of shear cracks and improved the damage resistance of beams. Compared with RC beams, the shear strength, displacement ductility factor, and deflection-clear span ratios corresponding to the peak load and ultimate deflection increased by up to 30.5%, 44.9%, 150.0%, and 148.0%, respectively. RHDC beams exhibited higher residual strength and deformation capacity than RC beams, thus indicating that HDC significantly improved the brittle shear failure mode. Specimens H-1 and H-2 exhibited the largest improvement in shear strength and displacement ductility factor, respectively, compared with RC beams. The shear strength of RHDC beams increased as the shear span to effective depth ratio decreased. For RHDC beams with the same shear span to effective depth ratio, the shear strength increased with the increase in the longitudinal reinforcement ratio and stirrup ratio under shear compression failure.

Experimental research on fatigue behavior of prestressed ultra-high performance concrete beams with high-strength steel bars

Ultra-High Performance Concrete (UHPC) is a new type of cement-based composite material that exhibits exceptional compressive strength, tensile properties and durability compared to ordinary concrete. Comprehensive research on the flexural fatigue behavior of prestressed UHPC beams is presented in this paper. The failure mode of the fatigue beam is the fatigue fracture of the tensile steel bar in the pure bending section. In the fatigue process, the fatigue behaviors of prestressed UHPC beams including crack width, midspan deflection, the strain of UHPC in compression zone and strain of high-strength steel bars exhibit two stages which are corresponding to damage initiation, steady damage propagation. The residual deflection at the midspan increases with the increase of fatigue cycles. The prestressed UHPC beam reinforced with high-strength steel bars shows stable structural behavior even after concrete cracking. The S–N expression of prestressed UHPC beam with consideration of the influence of stress amplitude of high-strength steel bars is proposed. The stiffness degradation formula of the fatigue beam and calculation methods of fatigue crack width and fatigue deflection are proposed.

Study on Durability and Piezoresistivity of Cement-Based Piezoelectric Materials Mixed with Carbon Fiber and Iron Tailings under Salt-Freezing Erosion

Under the complex working conditions in cold areas, in order to achieve health monitoring of engineering structures, carbon fiber and iron tailings sand were added to ordinary cement-based materials to prepare cement-based piezoelectric composites, and the deterioration of their pressure-sensitive properties and mechanical properties under the action of the sulfate-freeze-thaw cycle was studied. Six groups of specimens and a set of benchmark specimens were prepared according to different contents of carbon fiber and iron tailings sand, and the specimens of each group were analyzed qualitatively and quantitatively after 50, 100, and 150 freeze-thaw cycles. Based on the external damage analysis, it was concluded that with the increase in the number of freeze-thaw cycles, the apparent morphology of the specimens in each group continued to deteriorate. After 150 freeze-thaw cycles, the addition of a certain proportion of carbon fiber and iron tailings can improve the compactness of cement-based composites, effectively inhibit the development of cracks, maintain the integrity of the apparent morphology of the specimen, and the quality loss rate of the specimen does not exceed 5%. Based on the internal damage analysis, it is concluded that the specimen mixed with carbon fiber and iron tailings has undergone the freeze-thaw cycles, and its relative dynamic elastic modulus generally shows a trend of first rising and then falling, and after 150 freeze-thaw cycles, the relative dynamic elastic modulus of C04T30 specimen is 85.5%, and its compressive strength loss rate is 20.2%, indicating that its freeze resistance is optimal. The compressive stress and resistivity change rate of each group of cement-based piezoelectric composite specimens that have not undergone freeze-thaw cycles are approximately consistent with the linear attenuation relationship. Those that have undergone 150 freeze-thaw cycles approximately conform to the polynomial attenuation relationship. The correlation coefficient between the compressive stress and the resistivity rate of the change fitting curve are all above 0.9, and the correlation is high; therefore, the deterioration of the structural mechanical properties after freeze-thaw cycles can be reflected by the resistivity change rate. After 150 freeze-thaw cycles, the pressure sensitivity coefficient of the C04T30 specimen is 0.007294, which has good pressure sensitivity. So, cement-based piezoelectric composite material can be embedded as an impedance sensor to monitor the health of engineering structures.